The present invention relates to a normally white mode active liquid crystal display element which is transparent when no voltage is applied to its liquid crystal and a method for treating its defective pixels.
A description will be given first, with reference to FIG. 1, of a conventional liquid crystal display element, in which liquid crystal 14 is sealed in a space defined by a pair of opposed transparent base plates 11 and 12 as of glass with a spacer 13 interposed therebetween along their marginal portions. On the inside of the one transparent base plate 11 there are formed a plurality of pixel electrodes 15 and thin film transistors (hereinafter referred to as TFTs) 16 which are disposed adjacent to the pixel electrodes 15, respectively, and serve as switching elements. Each TFT 16 has its drain connected to the corresponding pixel electrode 15. On the inside of the other transparent base plate 12 there is formed a transparent common electrode 17 in opposed relation to the plurality of pixel electrodes 15. The transparent base plates 11 and 12 are deposited all over their exterior surfaces with polarizing films 10a and 10b, respectively. For example, in the case where the liquid crystal 14 is a 90.degree. twist nematic liquid crystal, the polarizing films 10a and 10b are disposed with their directions of polarization held at right angles to each other, for instance. With such an arrangement of the polarizing films 10a and 10b, when no voltage is applied to the liquid crystal 14, light having passed through the one polarizing film undergoes a 90.degree. rotation in its direction of polarization during the passage through the liquid crystal 14 and is allowed to pass through the other polarizing film, whereas when a voltage is being applied to the liquid crystal 14, the light does not undergo such a 90.degree. rotation in its direction of polarization and is intercepted by the other polarizing film. That is, the display element operates as a normally white mode element in this instance. Incidentally, when the polarizing films 10a and 10b are arranged with their directions of polarization held in parallel to each other, the display element will operate as a normally black mode element. In the case of a reflecting type display element, the exterior surface of the base plate 11 is coated with a reflecting metallic film in place of the polarizing film 10a.
As shown in FIG. 2, the pixel electrodes 15, substantially square in shape, are closely arranged in rows and columns on the transparent base plate 11. A gate bus 18 is formed adjacent to and extends along each row of the pixel electrodes 15 and a source bus 19 is similarly formed adjacent to and extends along each column of the pixel electrodes 15. The TFTs 16 are each disposed at or near the intersection of the gate and source buses 18 and 19. The TFTs 16 in each row have their gates connected to the corresponding gate bus 18 and the TFTs 16 in each column have their sources connected to the corresponding source bus 19. The TFTs 16 have their drains connected to the pixel electrodes 15 corresponding to them, respectively.
By applying a voltage across selected ones of the gate and source buses 18 and 19, only the TFT 16 supplied with the voltage conducts to store charges in the pixel electrode 15 connected to the drain of the conducting TFT 16 and then a voltage is applied across only that portion of the liquid crystal 14 between the pixel electrode 15 and the common electrode 17, thereby making that pixel of the liquid crystal display transparent or opaque to provide a selective display. The display can be erased by discharging the charges stored in the pixel electrode 15.
The TFT 16 has such a construction as shown in FIG. 4 which is a sectional view taken on the line IV--IV in FIG. 3. A light shield 21 is formed of a light intercepting metal such as chromium or molybdenum on the transparent base plate 11 at the position corresponding to the TFT 16 and an insulating layer 22 is formed all over the transparent base plate 11, covering the light shield 21. On the insulating layer 22 the pixel electrode 15 and the source bus 19 are each formed by a transparent electrode film as of ITO(i.e. indium-tin oxide). A semiconductor layer 23 as of amorphous silicon is formed, filling the gap between parallel-opposed portions of the pixel electrode 15 and the source bus 19. The semiconductor layer 23 is covered with a gate insulating film 24 as of silicon nitride. A gate electrode 25 is formed on the gate insulating film 24 in opposed relation to the semiconductor layer 23, one end of the gate electrode 25 being connected to the gate bus 18. Those portions of the pixel electrode 15 and the source bus 19 which are in contact with the semiconductor layer 23 constitute a drain electrode 15a and a source electrode 19a, respectively. The electrodes 15a and 19a, the semiconductor layer 23, the gate insulating film 24 and the gate electrode 25 make up each TFT 16. The gate electrode 25 and the gate bus 18 are simultaneously formed of, for example, aluminum. To safeguard the liquid crystal 14, a passivation layer 26 is coated almost all over the transparent base plate 11, covering the gate electrode 25. The light shield 21 is to prevent light from incidence to the semiconductor layer 23 which otherwise produces a photoelectric effect.
As shown in FIG. 5 which is a sectional view taken on the line V--V in FIG. 3, one end portion of the pixel electrode 15 is extended below the adjoining gate bus 18 to substantially the intermediate portion thereof to form an extended portion 15b, providing a capacitive portion 27 in the gate insulating film 24 between the extended portion 15b and the gate bus 18. The capacitance of the capacitive portion 27 supplements the electrostatic capacitance of the pixel electrode 15 and is effective in providing a large time constant in combination with the resistance value of a channel portion of the TFT 16.
In such an active liquid crystal display element of the type in which the pixels are transparent when no voltage is applied across the pixel electrodes 15 and the common electrode 17, that is, in what is called a normally white mode display element, a defective pixel always remains transparent, i.e. forms a white defect, whether or not a voltage is being applied across the corresponding pixel electrode and the common electrode. The white defect is more noticeable than a black defect which always intercepts light, and hence markedly degrades the display. Therefore, defective pixels of even such a number (or density) as to be allowable in a normally black mode liquid crystal display element (in which defective pixels always intercept light and hence remain black) would not be allowable in the normally white mode liquid crystal element, and consequently, the production yield of the normally white mode liquid crystal display element is appreciably low.